Description of the Related Art
The use of hollow threaded perforated interbody spinal fusion implants such as taught by U.S. Pat. No. 5,015,247 to Michelson ('247), incorporated by reference herein, is now commonplace. Nevertheless, because of the structure and biomechanical properties of these implants, the use of such implants has not been available for all patients requiring spinal fusion, but rather has been limited to a subset of that population. While such implants have proven to be very successful when used correctly, such success has not been universal.
A previously identified problem as discussed in U.S. Pat. No. 4,593,409 to Michelson ('409), incorporated by reference herein, is the frequent need for such implants to have a reduced combined width relative to their combined height. This permits the height, which is usually the implant diameter, to be sufficiently great so as to span the height of the distracted disc space and adequately penetrate and engage each of the vertebral bodies adjacent that disc space, and yet have a significantly lesser width so that when such implants are utilized in side-by-side pairs, the combined width is such that the paired implants do not protrude beyond the width of the spine. Historically, this not infrequent situation has deprived many patients needing spinal fusion from use of the prior art technology as implants of the desired height could not safely be placed within that patient's disc space because of the width problem. Alternatively, downsized versions of these implants were implanted with poor results as the implants were of insufficient size to adequately function for their intended purpose.
As discussed in Michelson '409, implants having various vertebral bone engaging surface projections have the advantage of enhanced stability within the spine as compared to an implant having a smooth surface. The use of a thread or thread portions has proven particularly beneficial and have been described in Michelson '247. As described in Michelson's co-pending application Ser. Nos. 08/484,928; 08/480,904; and 08/480,908 incorporated by reference herein, similar devices in which opposite vertebrae engaging arcuate surfaces are in angular relationship to each other may be useful to be fuse the vertebrae in a more lordotic angular relationship relative to each other. Implants of the related art are taller near the end adapted to be placed proximate the anterior aspect of the vertebral bodies than at the opposite end adapted to be placed proximate the posterior aspect of vertebral bodies. The related art implants are generally wedge-shaped when viewed from the side. The wedged configuration causes the implant to be less stable within the spine than if it were non-wedged. Further limiting the stability of these implants, compromising the surface area available for contact and fusion, and limiting the volume of osteogenic material containable within the implants is a result of the fact that these implants have generally been relatively flat across their trailing ends so as to be rotationally symmetrical about their mid-longitudinal axes. The anterior aspects of the vertebral bodies are generally curved from side-to-side. As a result, related art implants needed to be rather deeply inset into the disc space and away from the anterior aspects of the vertebral bodies so as to prevent the implants from protruding from the disc space at their lateral wall and trailing end junctions, wheresuch a protrusion of the implant could place vital structures adjacent the spine at risk.
There is, therefore, a need for further improvement in the design of such interbody spinal fusion implants so as to firstly extend their range of usefulness, and secondly to further increase the rate of success when such implants are used.